The invention relates to a drive for a continuously operating screw ejection centrifugal separator having an epicyclic gear train between the centrifuge drum and the ejector screw.
Such drives for screw ejection centrifugal separators are known, the centrifuge drum being driven directly by the motor and the ejector screw being driven by an epicyclic gear train, such as a planetary drive. It is unimportant whether the rotatory speed of the screw is greater or less than that of the centrifuge drum, as long as the relative movement between them produces a transport of the solids being separated in these separators towards the smaller end of the drum. The friction that is to be overcome by the screw has a tendency to reduce the differential rotatory speed between the drum and the screw, i.e., to drive the screw at the same speed as the drum. This torque depends on the nature and rate of flow of the material being separated, its solid content, and the length of the dry zone in the separator.
Screw ejection centrifugal separators are therefore often advantageously equipped with drives which are torque-controlled or with timed drives whenever the raw liquids to be separated vary in rate of flow and in solid content, and when the solid matter must satisfy special requirements with regard to residual moisture. The regulation and control of the separator are accomplished in this case by varying the rate of product input and/or the differential speed.
In known planetary drives, the input shaft of the gear train can be held and the applied torque measured, but variation of the differential speed during operation is not possible. Any necessary change in the differential speed would have to be accomplished by changing the transmission ratio in the gear train.
German Offenlegungsschrift No. 24 32 284 discloses a drive for continuously operating screw ejection centrifugal separators, in which the regulation of the differential speed between the screw and the centrifuge drum is accomplished through a hydrostatic drive, the screw being driven by a hydraulic motor revolving at the drum speed and the differential speed being dependent upon the volume of oil delivered to the hydraulic motor by a controllable pump. The reference magnitude for the control is the torque-related oil pressure.
Such hydrostatic drives are suitable especially for small and medium screw ejection separators. In the case of larger screw ejection separators, relatively high differential speeds cannot be achieved. The drive requires additional maintenance, and contamination due to leakage in the high pressure unit is not avoidable. Such drives are furthermore expensive and require a relatively great amount of space.
German Pat. No. 10 40 460 discloses the control of the speed of a screw ejection centrifugal separator by means of an eddy current brake in which the input shaft of the gear train is slowed by braking until the desired differential speed is reached. Within given limits, the slip and with it the differential speed can be kept constant by appropriate electrical manipulation at varying loads. If the limit is exceeded due to excessively high torque, an increase in the differential speed can be accomplished by varying the slip.
Rotatory speed control through an eddy current brake is also suitable only for smaller screw ejection centrifugal separators. In the transmission of higher torques, however, higher drive ratios are required, even though they diminish the range of control. Relatively great power losses develop, especially at the lower differential speeds, and explosion-proof separator designs are not possible.
Furthermore, screw drives are known in conjunction with controlled-speed motors, in which a continuous adjustment of the differential speed is possible, as well as a torque-related variation of the differential speed and/or of the product input. Such drives, however, are very expensive.